Increasing adipose cell size in the aging rat model of obesity is accompanied by an increasing basal glucose transport activity and a decreasing ability of insulin to stimulate glucose transport activity in the intact isolated adipose cell. The ability of insulin to increase the number of functional glucose transport systems in this cell's plasma membranes, as assayed by D-glucose-inhibitable 3H-cytochalasin B binding, is also decreased. Since insulin appears to enhance glucose transport by triggering the translocation of transport systems to the plasma membrane from an intracellular pool which is associated with a fraction enriched in membranes of the Golgi apparatus, the effects of cell size on the number of transport systems in this pool and the total number of transport systems per cell have been examined. The markedly enlarged cells prepared from 800 g rats contain, at most, 50% more transport systems per cell than small cells from 180 g rats. Furthermore, even in the basal state, the number of transport systems in the enlarged cell's plasma membranes is already increased and this cell's intracellular pool is already depleted.